In LTE (Long Term Evolution) and an evolved version thereof, i.e., LTE-A (LTE-Advanced) formulated in 3GPP (3rd Generation Partnership Project), uplink is provided employing SC-FDMA (Single-Carrier Frequency-Division Multiple Access) with a small PAPR (Peak-to-Average Power Ratio) and high power usage efficiency in a terminal (for example, see Non-Patent Literature (hereinafter, abbreviated as NPLs) 1 to 4). In uplink of LTE and LTE-A, scheduling for allocating time and frequency resources according to the propagation path environment of the terminal, and adaptive control for controlling a coding rate or a modulation scheme are performed. In order to appropriately perform frequency scheduling and adaptive control to enable high throughput, it is indispensable for a base station side to know the propagation path situation of the terminal.
In order to measure the uplink propagation path situation of the terminal, an SRS (Sounding Reference Signal) is used in uplink of LTE (NPL 1). An SRS is a reference signal transmitted with the last SC-FDMA symbol of an uplink subframe (PUSCH: Physical Uplink Shared Channel) including a plurality of SC-FDMA symbols. A base station can know the uplink situation according to CSI (Channel State Information) calculated using the SRS or the reception quality of the SRS.
LTE employs a P-SRS (Periodic-SRS) transmitted periodically at time indicated by an instruction from a higher layer, such as RRC (Radio Resource Control) information. The base station beforehand instructs the terminal on the transmission subframe for an SRS, the period thereof, the power offset for the SRS to be transmitted, the frequency bandwidth, the frequency position, and an orthogonal resource, such as Comb or CS (Cyclic Shift) for orthogonalization to an SRS of another terminal. The terminal transmits an SRS with the last SC-FDMA symbol in the instructed subframe. In this way, regardless of the presence or absence of transmission of data and a control signal in uplink, the base station can periodically measure CSI of the terminal.
Meanwhile, uplink packet communication generally has high burstiness. It is preferable for a base station to be able to measure CSI on a necessary band when needed. Moreover, even when performing no data communication in uplink or downlink, a terminal periodically transmits an SRS and therefore consumes extra power. For this reason, LTE-A employs an A-SRS (Aperiodic-SRS) to be transmitted on the basis of a transmission request included in DCI (Downlink Control Information) which is a control signal indicating data assignment in uplink and downlink. An A-SRS is transmitted only upon request. This can reduce unnecessary power consumption in the terminal, also reduce interference to and from another cell and improve the efficiency of SRS resources.
Note that, there has been a discussion on the introduction of a heterogeneous network (HetNet) in which a plurality of base stations (hereinafter referred to as nodes) providing different coverage areas are deployed in a cell in LTE Release 11 (hereinafter referred to as Rel. 11), which is a further evolved version of LTE-A. A HetNet enable, for example, reception in a receiving node with a small path loss and offload for traffic, and therefore enables high throughput. Moreover, a terminal can decrease transmission power for a receiving node with a small path loss and can therefore reduce power consumption. For these reasons, in comparison with a non-HetNet involving only a macro node, a HetNet can improve a transmission speed while reducing necessary transmission power for a terminal.
Moreover, CoMP (Coordinated Multi-Point) in which these nodes transmit and receive a signal in cooperation has also been discussed in Rel. 11 HetNet (NPL 4). FIG. 1 illustrates an example of a HetNet CoMP system. HetNet CoMP includes one or more macro base stations (macro nodes), one or more pico base stations (pico nodes), and one or more terminals. CoMP can enhance an SNR (Signal-to-Interference plus Noise Power Ratio), for example, by a plurality of nodes receiving and combining signals transmitted by a terminal that is located at a cell edge and is strongly influenced by interference. Moreover, nodes can transmit and receive in a coordinated manner. Therefore, optimal nodes can be used independently in uplink and downlink. For example, a PDSCH is preferably transmitted by a node maximizing the reception power in a terminal, and a PUSCH is preferably received by a node minimizing a path loss. Introduction of CoMP enables communication with different nodes in uplink and downlink. This prevents large differences in the throughput and quality between uplink and downlink.
In order to acquire the effect of HetNet CoMP, it is important to appropriately select transmitting/receiving nodes participating in communication from among nodes distributed geographically, and to appropriately switch between nodes according to a peripheral situation or the situation of the terminal Transmitting/receiving nodes may be selected and switched using a reference signal (for example, CRS, CSI-RS, or SRS) transmitted in uplink and downlink. In this case of using a CRS or a CSI-RS transmitted in downlink, a terminal measures CSI to each node and feeds back the CSI using uplink. Then, the base station side determines a transmitting/receiving node on the basis of the fed-back CSI. On the other hand, in the case of using an SRS transmitted in uplink, the base station side can directly measure CSI with an SRS transmitted by the terminal. Therefore, the system using an SRS can decrease the amount of information fed-back from the terminal to the base station in comparison with a system using a CRS or a CSI-RS. Moreover, the time required from measurement of the terminal to the completion of the feedback is omissible, so that a feedback delay can be reduced.
It is known that the reversibility of a channel is satisfied in TDD (Time-division duplex), and that downlink precoding, the scheduling of a PDSCH, or adaptive control is possible on the basis of the CSI measurement result acquired using an SRS. HetNet CoMP involves a high probability of enabling communication with a node having a small path loss since a plurality of nodes are distributed in the cell. Therefore, it can be said that there is a high possibility of also using downlink adaptive control using an SRS.
As described above, in and after Rel. 11 into which HetNet and CoMP are introduced, an SRS may be used for various purposes such as not only uplink scheduling and adaptive control used in the related art, but also selection of a transmitting/receiving node and downlink adaptive control.